1. Field of the Invention
This invention relates to a near infrared absorption filter having a high transmission in the visible region and a sharp absorption at about 600 to about 700 nm.
2. Description of the Prior Art
In color television camera tubes, the spectral sensitivity of the photoelectric element extends to the infrared region. Unless the infrared light is cut off, good color reproduction cannot be obtained. Hence, an infrared absorption filter is required.
A conventional infrared absorption filter comprises a heat-absorbing filter utilizing Fe.sup.2+ ion coloration and a multi-layered coating formed thereon to permit relatively sharp absorption of light having a wavelength of about 600 nm or more. The filter is very expensive, and the spectral characteristics thereof fluctuate over a fairly broad range.
It is generally said that unless a filter has a transmission at 600 nm of about 75% or more and a transmission at 700 nm of about 15% or less, color reproduction will be aggravated. Conventional near infrared filters containing copper do not have such sharp absorption characteristics and have low transmission at 600 nm when their transmission at 700 nm is 15%. Hence, such cannot be used as infrared absorption filters for color television cameras.